Seismic Performance of Axially Restrained Reinforced Concrete Frame Beams

Abstract

Reinforced concrete beams tend to elongate after flexural cracking and yielding; however, the elongation is restrained by the surrounding structural components in a RC moment frame. Experiments were conducted on seven 1/2-scale interior beam-column subassemblies to study the effects of axial restraint on the seismic performance of RC frame beams and beam-column joints without the presence of floor slabs. The test setup permitted applying axial restraint to beam ends and measuring the compressive axial force passively generated in the beams. Major test variables included beam flexural reinforcement ratio and axial restraining rigidity. Without axial restraint, the total beam elongation reached 3.75% of the beam height at 3% lateral drift. Under the considered levels of axial restraining rigidity, large compressive axial force developed in the beams, leading to an axial force ratio up to 0.25. The axial restraint increased both beam flexural stiffness and strength. Depending on the tensile reinforcement ratio, beam flexural capacity increased 40%–150% at 3% drift. Compared with the unrestrained specimens, the axially restrained specimens suffered greater damage in the beam plastic hinge regions and beam-column joints due to the dramatically increased shear demand, which can negatively impact the seismic performance of a RC frame.